A variety of core-molding techniques have been employed for manufacturing cast products from molten materials. The Background section and the claimed invention of U.S. Pat. No. 3,940,102 outline several such techniques involving, respectively, collapsible and withdrawable cores in the manufacture of hollow fusion-cast refractories. Another collapsible coring technique for fusion-casting of molten carbide or oxide materials analogous to fusion-cast refractories is shown in U.S. Pat. No. 3,506,235. Collapsible and withdrawable cores are also known for the manufacture of hollow metal castings, e.g. see U.S. Pat. Nos. 1,946,451, 1,698,308 and 1,683,475. Withdrawable cores are even known for manufacturing glass articles from molten glass as shown in U.S. Pat. No. 1,929,842.
The general problem requiring such specialized coring techniques involves the fact that cracking or hot tearing occurs in a cooling solidified casting as it shrinks around and onto a core that does not collapse or is not withdrawn to accomodate such cooling shrinkage of the solidified casting and to avoid the resultant detrimental strain in such casting. Since fusion-cast refractories (of nonmetallic oxides, carbides, borides and the like) are generally much more brittle materials than cast metals, the problem is especially acute with such refractories.
In recent years, single-piece tap hole blocks for steelworking furnaces and vessels have been made as some of the more notable hollow fusion-cast refractory products. Commonly these articles have been made with collapsible cores of baked, organic-bonded refractory grain. Those cores fail to give adequate highly quenched microstructure of fine interlocking crystals in the refractory solidified around them. They also tend to cause interconnecting porosity in such refractory as a result of gases given off from those cores when their organic bond becomes highly heated by the surrounding solidifying refractory. In fabrication these cores are semi-fused to the refractory which requires expensive diamond drilling resulting in exposure of said porosity. These dificiencies, in particular the exposed porosity, significantly limit the service life of the blocks under the severely abrasive, erosive and corrosive environment of thousands of tons of molten steel passing through their orifices. Thus, a strong need existed for making the fusion-cast blocks, at least in those portions forming and underlying the orifice surfaces, with a highly quenched microstructure and a very dense macrostructure.
Conceptually the meltable metal cores of U.S. Pat. No. 2,004,378 may seem to provide adequate mass and heat capacity to absorb heat from molten refractory contacting them so as to yield the needed quenched, dense structure in the adjacent solidified refractory prior to melting of such core. However, as a practical matter such cores are not always able to commercially provide the needed saleable hollow fusion-cast refractory product because of several key factors. Sizes are restricted because of the economic consideration of the material of core construction and by the critical requirement of balancing the mass of the core to the mass of fusion-cast refractory so that the core either melts too soon thereby leading to "break-out" of molten refractory into the core space or does not adequately melt causing strain and cracking of the fusion-cast article. Very importantly involved with solid metallic cores in extreme high tempertaure applications is the detrimental effect of possible stream impingement upon the core resulting in premature melting of certain areas of the core thus yielding a defective non-saleable final product. Thus, there are cases where the required core size and the required fusion-cast refractory size are not compatible for making a useful product.
The withdrawable graphite core of U.S. Pat. No. 3,940,102 provided adequate mass and heat absorbing capacity for producing the needed dense quenched structure, but it was found to be awkward and non-economical to commercially operate consistently at the right time to avoid cracking of the fusion-cast refractory.
Employment of many separate core segments, individually of small mass and spaced around a central column as shown in U.S. Pat. No. 1,946,451, generally have inadequate heat absorbing capacity such that these cores - whether of metal or even graphite - would fail to produce adequate depth of highly quenched microstructure in the fusion-cast refractory product and/or would be damaged by interaction with the cooling solidified refractory surrounding them, which interaction would also likely damage or contaminate such refractory. With segments of small mass the possibility of violent reactions when in intimate contact with certain molten materials is significantly increased resulting in compromise of safety and/or an unsaleable product.